1. Field of the Invention
This invention relates to Bolted Flanged Connections (BFCs) with gaskets as a sealing elements that have to provide a leak-tight joint and extended service life of plant/piping systems such as reactors, steam generators, heat exchangers, boilers, pressure vessels and piping, and others engineering structures that operate under critical operational conditions including high internal pressures and a variety of operational temperatures.
2. Prior Art
Bolted Flanged Connections are one of the most popular ways to assemble different parts of engineering structures. For many reasons this type of connection should be considered as unique means to create specific mechanical clamping forces and ensure a durable and leak-tight joint between adjacent members of pressure vessels and piping systems providing their simple assemblage, disassemblage and reassemblage.
From structural integrity point of view safe design of BFCs has been solved and standardized, but joint leakage events remain currently an unresolved problem and a main cause of bolt-flange-gasket damage and failure. The leakage problem is especially important for plant/piping systems used in aerospace, submarine shipbuilding, petrochemicals, petroleum refining, fossil fuel and nuclear power generation, and other industries. As a rule, these systems operate under critical conditions including high internal pressures and a variety of operational temperatures. Experimental and analytical investigations of BFCs have revealed that joint opening followed by leakage results from decrease of gasket contact stress due to creep relaxation of bolted fasteners and gaskets under critical operational conditions accompanied by flange flexion with rotation due to internal pressure.
Industry experience with continuing flanged joint leakages has demonstrated that leakage events are attributed to high rate of corrosion of bolt-flange-gasket systems that is combined with high level of stresses and strains due to internal pressures, flow-induced vibrations, elevated temperatures, integral flow of neutrons, and other critical factors. Statistic data show that, for example, piping system leakages alone conservatively cost each process industry hundreds millions of dollars annually in lost profits as a result of plant shutdowns, production penalties, maintenance rework activities, and equipment repair or replacement. It is necessary to add enormous material and financial losses due to possible fires, explosions, environmental pollution, and some other disasters. Hence, the leak tightness has a greater influence on safe service life of BFCs, and highest priority in plant reliability programs is to limit or exclude the leakages, thus protecting critical engineering structures from untimely degradation and failure.
A wide range of patent documents is dedicated to flange and gasket design improvements in order to increase leak tightness of BFCs. Most of them relate to different flange designs and sophisticated gasket materials and styles that should improve the interaction of the bolts, flanges and gaskets in order to prevent operational leakages. It is impossible to describe all patent documents relating to BFCs' improvement, but the closest of them to the present invention are the followings.
The BFC described by DE Pat. No. 64013 to Schwoerer has a small clearance between adjacent flange faces located at the periphery of the flanges and bridged by the bolt preload force to create a clamping of lens-shaped gasket. Same approach is used in DE Pat. No. 124715 to Janke and GB Pat. No. 2200179 to Porter.
DE '715 discloses superimposed annular flanges having a small clearance at the periphery of flanges that is bridged by the bolt preload force to clamp a gasket compressed between two tube ends.
GB '179 describes a flanged joint having a small clearance at the periphery of flanges that is closed when the bolts are correctly and uniformly tightened forming initial joint between inner flat faces of the flanges by means of metal-to-metal contact.
All three patent documents reproduce on the whole a conventional approach to the flange design based on application of raised-face or flat-face flanges. The only difference that is disclosed in GB '179 consists in possibility to prevent bolt overstressing during the bolt tightening followed by internal pressure and external loading.
Next attempts to improve flange design in relation of leak tightness increase are contained in U.S. Pat. No. 2,412,487 to Amley, FR Pat. No. 1024183 to Syndicat Dauphinois, GB Pat. No. 1210291 to Haworth, U.S. Pat. No. 3,135,538 to George and U.S. Pat. No. 3,771,817 to Schnabel.
All these documents disclose the means to mechanically change a shape of adjacent flanges in order to increase compression of the gaskets or other sealing elements. The general approach used in cited patent documents consists in fabrication of frusto-conical faces of adjacent flanges having a hollow space to place the gasket or other sealing element that is further compressed during the bolt preload.
The GB '291 entitled “Metal-to-metal joint” discloses a sealing joint between two members which comprise metals of different elastic limits. The member having lower elastic limit is adapted to form the area of contact by means of plastic deformation of its initial frusto-conical shape during the bolt tightening.
U.S. '817 describes a similar approach to join two mutually braced metal parts of the pipes having a covering of plastic material. The metal parts have annular clamping flanges of frusto-conical shape with the hollow space adapted to extend parts of plastic material. The flanges have a free peripheral edge for compressing the plastic material between annular clamping faces in hollow space, so that the plastic material may flow into the hollow space to provide a perfect seal during the bolt tightening. The similar approach is described in U.S. Pat. Nos. 2,412,487 and 3,135,538.
FR '183 proposes a sealing joint with flanges of sophisticated shape having frusto-conical parts and internal annular cavities that facilitates flange deformations during the bolt tightening to provide a contact stress distribution favorable for leak tightness.
A common weakness of all cited above prior patent documents is a general approach to form the flange design by means of raised-face flanges which will be inevitably subjected to rotation around a gasket surface due to internal pressure that will create conditions favorable for early joint opening followed by leakage. Moreover, the problem of creep relaxation of bolted fasteners and gasket remains out of consideration.
Generally, creep is accompanying by stress relaxation, and elongation of the bolts along with contraction of the gasket due to creep is a very serious problem because it leads to bolt load and gasket stress losses that, in turn, increase the leakage rate. A plant maintenance practice includes periodical retightening or replacement of the bolts and gaskets subjected to extended creep relaxation to prevent leakages, and, having in mind millions of bolts and gaskets used in critical technological equipment, this procedure involves an expensive time-consuming process and provides only temporary effect because the creep relaxation increases rapidly after each additional retightening, and risk of leakage events or connection blowout relatively increases. Nevertheless, during the development of must design procedures little consideration has been given to the creep relaxation of bolted fasteners and gaskets, and similar situation is observed with existing patent documents.
U.S. Pat. No. 6,199,453 to Steinbock entitled “High temperature bolting system” offers a sophisticated apparatus for maintaining a clamping force between component parts of a steam turbine while operating at temperatures from 800 DEG. F. to 1200 DEG. F. However, the disclosed elongated stepped fastener shank manufactured from superalloy Inconel 718 having a thermal expansion coefficient similar to flange material and creep strength which is several times greater than creep strength of flange material cannot stop a creep relaxation process and protect proposed bolting system from bolt elongation increase and gasket stress decrease. Moreover, the high level of stresses and high operational temperatures will induce the high level of creep relaxation of the bolts and gaskets.
U.S. Pat. No. 4,913,951 discloses a method of fabrication of reinforced polytetrafluoro -ethylene (PTFE) gasketing materials “characterized by high strength, excellent recovery and superior creep relaxation resistance”. This material is proposed for use in BFCs. However, these super characteristics did not exclude the creep relaxation from 20% to 30% depending on thickness of proposed tested materials. Moreover, the tests were carried out under standard procedure during only 22 hours at only 212 Deg. F. Thus, the proposed gasketing materials demonstrate “superior” physical and functional properties when compared with previous PTFE gasketing materials described in prior art.
The most important failing, however, is the fact that proposed gasketing materials copy a typically used approach to the fabrication of sealing elements based on traditional “passive” behavior under critical operational conditions of all known today gasketing materials excepting those described in U.S. Pat. No. 5,226,683 to Julien et al. and in U.S. Pat. No. 6,435,519 to White. These two documents are the first attempts to introduce the new gasketing material fabricated from NiTi (Nitinol) Shape Memory Alloy (SMA).
U.S. '683 discloses a method to use a gasket of Nitinol SMA under its martensite state to fill the space between the hard flange faces having microscopic surface irregularities that can prevent the fluid leakage between the faces and will allow further to reuse the gasket. The Nitinol SMA from which the gasket of this invention is fabricated “remembers” the gasket shape which it had when it was last formed in its austenite state. When this gasket is deformed under temperature of martensite state it fills the irregularities of flange faces under pressure exerted by hard clamping members of the flanges.
The shape memory effect is used when gasket resumes its original shape after heating to austenite state during the restoration step after release and before reuse. Although this invention has failed a main problem of leakage reduction by means of creep inhibition, it remains a turning point to use advanced shape memory materials as a sealing elements of BFCs.
U.S. '519 represents a next attempt to use the Nitinol SMA as a gasketing material to provide a seal between component parts of imaginary generalized assembly. Unfortunately, this invention claims a well-known procedure to clamp the gasket between adjacent flange faces. As for application of gasket of Nitinol SMA, this invention claims the spring forces generated by bending of the gasket when it is in super-elastic state. However, the shape memory alloys in super-elastic state display all mechanical properties of typical elastic materials including the property of creep relaxation while subjecting to elevated temperature and external loading. Meanwhile, this invention tries to open a real way to the application of shape memory alloys as sealing materials even though the problem of creep relaxation and leakage elimination remains out of consideration.
U.S. Pat. Nos. 3,759,552, 4,001,928, 4,149,911, 4,198,081, 4,281,841, 4,450,616, 4,469,357, 4,501,058, 4,537,406, 5,791,847 as well as GB Pat. Nos. 1554432, 1580036, SU Pat. No. 1086282 and JP Pat No. 62-116292 describe the means of stressing a structural members of Nitinol shape memory alloy components that provide stiffness to shell structures and tubular members as well as the means of stressing head bolts or other prestressed fasteners. These documents, however, did not touch the problem of creep relaxation of mechanical fasteners and gasketing joints.
Japanese Pat. No. 62-188764 describes a method to manufacture a bolt of Nitinol SMA that may be easily fastened and detached. This bolt is subjected to axial compression and aging treatment under specific temperatures while holding it under compressive strain. Thus-obtained bolt reversibly repeats the elongation in a length direction at initial temperature of martensite transformation and the contraction at initial temperature of reverse transformation. Owing to these characteristics, the bolt length is arbitrary changed, so that bolt may be firmly fastened or easily detached. The procedure of bolt manufacturing relates to known method of shape memory formation by means of stress-induced martensite that is obtained under conditions of fixed shape at temperature higher than temperature of austenite state of shape memory alloy followed by aging and unloading under temperature lower than temperature of martensite state. This procedure is very complex for a compressed bolt because of possible bolt's buckling, and moreover, the problem of creep relaxation of the bolts is failed too.
None of the above-mentioned prior patent documents touch the problem of flange rotation and creep relaxation of the bolts and gaskets that have a major influence on operational leakage events. Accordingly, it is an object of the present invention to form a novel type of BFC with feature to inhibit flange rotation and creep-relaxation of bolted fasteners and gaskets due to critical operational conditions including high internal pressures and a variety of temperatures.